docs: def lookup(self, module_name):
          """Searches for a file providing given module.
  
          Returns the normalized module id and path of the file.
          """
          for search_path in self._paths:
              module_path = os.path.join(search_path, module_name)
              new_module_name, module_file = self._lookup(module_path, module_name)
              if module_file:
                  return new_module_name, module_file
          return None, None